The present invention relates to the testing of a fluid stream and more particularly relates to a method and apparatus for automatically testing a production stream of crude oil, water, and gas from a well to determine the percentages of each component in the stream.
A production stream from an oil producing well is normally comprised of a mixture of gas, crude oil, and water. Since royalty payments are based on the crude oil produced, it is necessary not only to meter the total volume of fluid produced from a well, but also to determine what percentage of produced fluid is crude oil. This normally is done by carrying out periodic laboratory tests on samples of the production stream taken at or near the wellhead.
The testing of a production stream is usually time consuming and may be complicated by (1) the composition of the crude oil itself, (2) small production rates, and/or (3) high water contents. For example, the production from the Belridge and San Ardo Fields in California is from heavy, viscous oil reservoirs which are produced with the aid of steam injection and in-situ combustion. The production streams from these wells are typically ones having highly viscous crude, a high water cut, and some gas. Further the production rates of the individual wells vary over a wide range, e.g. from 3 barrels per day to several hundred barrels per day. The oil contents vary from 2-50% and temperatures range from 50.degree.-200.degree. F.
Several procedures have been proposed and/or used for testing production streams of the type described above. At Belridge, one such procedure commonly called "3 Way AWT" (Automatic Well Tester) involved flowing a test stream from a well into a relatively large horizontal pressure vessel wherein the produced fluid was allowed to separate into gas, oil, and water layers. The free gas was vented and the oil/water and free water were independently measured as they were returned to the main production line through separate lines. Due to problems with (a) maintaining the oil/water and free water interface (essential for proper operation), (b) the unreliable dump valve controls, and (c) sand buildup, this testing procedure was found to give erratic data in Belridge and was discontinued after short use although it is still in use in other areas.
Another method, commonly called "open tank AWT", was also tested at Belridge and consequently rejected. This procedure involves flowing the production fluid through a test line into a large open tank, i.e., 100-barrel tank, where it is allowed to accumulate for the entire well test period. Under atmospheric pressure, free and solution gas escapes out of the fluid and is removed from the top of the tank. The oil and water are pumped from the tank at the end of the test period through an outlet line to the main production line in which an inserted flowmeter, in conjunction with a net oil computer, determines the oil and water production of the well during the test period. This procedure has a number of drawbacks involved in its use. First, it requires substantial surface area in the proximity of the well test manifold to accommodate the large tank (e.g. 100-barrel) and the associated pump station. Also, it has been found that this technique creates significant flow surges downstream at the associated oil treating facilities. Another major problem arises in handling the tested crude oil due to the increased viscosity of the heavy crude caused by the cooling down of the crude as it sets in the open tank. Further, this procedure may sometimes give a distorted picture about a particular well's performance due to the different back pressure when pumping into the main production line as against testing into the open tank. And last, due to the large volume (100 barrels) required for testing high producing wells, the test results on low producing wells, e.g. 3 barrels per day (126 gallons per day), are unreliable or impossible to obtain.